1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device in which column spacers for a cell gap are arranged between gate and common lines to reduce a contact area between the column spacers and an opposing substrate, and a stable cell gap is maintained over the whole panel by reducing variation of a thickness per area of a thin film transistor (TFT) substrate corresponding to the column spacers.
2. Discussion of the Related Art
A related art LCD device will be described with reference to the accompanying drawings.
FIG. 1 is an exploded perspective view illustrating a related art LCD device.
As shown in FIG. 1, the related art LCD device includes a first substrate 1, a second substrate 2, and a liquid crystal layer 3 formed between the first and second substrates 1 and 2 by injection.
In more detail, the first substrate 1 includes a plurality of gate lines 4 arranged along a first direction at fixed intervals, and a plurality of data lines 5 arranged along a second direction perpendicular to the first direction at fixed intervals. A plurality of pixel electrodes 6 are arranged in a matrix-type configuration within pixel regions P defined by crossing the gate and data lines 4 and 5. The first substrate 1 also includes a plurality of TFTs T formed at respective areas where the gate lines 4 cross the data lines 5 to transmit data signals from the data lines to the respective pixel electrodes according to gate pulse signals supplied to the gate lines.
Also, the second substrate 2 includes a black matrix layer 7 that blocks light from portions of the first substrate 1 except the pixel regions P, an R/G/B color filter layer 8 for displaying various colors, and a common electrode 9 for producing the image on the color filter layer 8.
In the aforementioned LCD device, the liquid crystal layer 3 is aligned by an electric field generated between the pixel electrode 6 and the common electrode 9. Light irradiated through the liquid crystal layer 3 may be controlled by the alignment direction of the liquid crystal layer 3, thereby displaying the image.
This kind of LCD device is referred to as a twisted nematic (TN) mode LCD device, which has disadvantageous characteristics such as a narrow viewing angle. In order to overcome the narrow viewing angle, an In-Plane switching (IPS) mode LCD device has been actively developed.
In the IPS mode LCD device, a pixel electrode and a common electrode are formed in a pixel region parallel to each other at a fixed interval therebetween, an electric field parallel to the substrates develops between the pixel electrode and the common electrode, thereby aligning the liquid crystal layer according to the electric field parallel to the substrates.
Meanwhile, a method of manufacturing a related art LCD device is classified into a liquid crystal injection method and a liquid crystal dispensing method depending upon a method of forming a liquid crystal layer between a first substrate and a second substrate.
First, the method of manufacturing the LCD device according to the liquid crystal injection method will be described as follows.
An LCD panel and a container having liquid crystal material therein are provided in a chamber, and the chamber is evacuated. Moisture and air bubbles in the liquid crystal material and the container are simultaneously removed, and an inside space of the LCD panel is maintained in a vacuum state.
Then, an injection inlet of the LCD panel is dipped into the container having the liquid crystal material in the vacuum state, and the vacuum state inside the chamber is brought to atmospheric pressure. Thus, the liquid crystal material is injected into the LCD panel through the injection inlet by a pressure difference between the inside of the LCD panel and the chamber.
However, the method of manufacturing the LCD device by the liquid crystal injection method has the following disadvantages.
First, after cutting a large sized glass substrate into the respective LCD panel regions, the injection inlet is dipped into the container having the liquid crystal material while maintaining the vacuum state between the two substrates. The process of injecting liquid crystal material between the two substrates is slow thereby lowering yield.
Also in the case of forming a large sized LCD device, it is difficult to completely inject the liquid crystal material into the inside of the LCD panel, thereby causing failure due to incomplete injection of the liquid crystal material.
Furthermore, the process of the liquid crystal material is slow and a large space is required for the liquid crystal injection device.
In order to overcome the problems with the liquid crystal injection method, the liquid crystal dispensing method has been developed. In the dispensing method, two substrates are bonded to each other after dispensing liquid crystal material on any one of the two substrates.
In this method, it is impossible to use ball spacers for maintaining a cell gap between the two substrates since the ball spacers move in the spreading direction of liquid crystal material.
Thus, instead of the ball spacers, patterned spacers or column spacers are fixed to any one of the two substrates to maintain the cell gap between the two substrates.
FIG. 2A is a structural sectional view illustrating a color filter substrate provided with column spacers, and FIG. 2B is a structural sectional view illustrating the state when a TFT substrate is bonded to the color filter substrate.
As shown in FIG. 2A, the column spacers 20 are formed on a black matrix layer (not shown) of the color filter substrate 2. In this case, each of the column spacers 20 has a height “h”.
As shown in FIG. 2B, if the color filter substrate 2 provided with the column spacers 20 is arranged to oppose the TFT substrate 1 and is bonded to the TFT substrate 1, the column spacers 20 are contracted at a height “h′” representing a height of a cell gap due to pressure generated when bonding the substrates together.
As shown in FIG. 2A and FIG. 2B, the column spacers 20 within a panel 10 bonding the substrates together are pressed at a thickness h-h′ according to the pressure during bonding and contracted at the height “h′” of the cell gap.
As described above, the LCD device including the column spacers has a relatively large contact area with an opposing substrate (TFT substrate) unlike the ball spacers, thereby causing a great frictional force due to the increase in the contact area to the substrate. Accordingly, in a case where the screen of the LCD device having the column spacers is rubbed, spots may be generated on the screen for a long time.
FIG. 3A and FIG. 3B are a plan view and a cross-sectional view illustrating the spots generated on the screen by touching the LCD panel.
As shown in FIG. 3A, if the LCD panel 10 is continuously touched with a finger or pen along a predetermined direction, the second substrate 1 of the LCD panel 10 is shifted at a predetermined interval along the touched direction as shown in FIG. 3B. At this time, the second substrate 1 is not restored to the original state for a long time. In this case, liquid crystal molecules 3 touched with a finger or pen are dispersed and the liquid crystal molecules 3 are gathered in the region around the touch portion. For this reason, a cell gap h1 corresponding to the region where the liquid crystal molecules are gathered is higher than a cell gap h2 of the remaining portions, thereby generating a touch defect that represents spots around the touch portion due to surplus and shortage of the liquid crystal molecules.
The touch defect is generated in the LCD device provided with the column spacers because the column spacers are fixed to one substrate and are in contact with another opposing substrate in a surface type to form a relatively large contact area unlike the globular ball spacers.
Consequently, the aforementioned related art LCD device has the following disadvantages.
First, in the related art LCD device provided with the column spacers, when the LCD panel is touched while being pressed in a predetermined direction, the substrates shifted in opposing directions are not restored to the original state or it takes a long time even if they are restored to the original state. For this reason, light leakage occurs in a portion where the liquid crystal molecules are pushed from the touch portion during a restoring time. It is noted that such a touch defect is caused by the frictional force generated by a large contact area between the column spacers and their opposing substrate.
Second, the ball spacers are dispersed for injection of the liquid crystal and have a globular form. When a predetermined area of the panel is pressed, the ball spacers corresponding to the predetermined area of the panel are slid and have tolerance to the pressed pressure. However, since the column spacers are selectively in portions except for the pixel region, the column spacers may easily be deformed when the portion having no column spacers is pressed. A structure below the column spacers may be broken.
Third, a height difference occurs in the column spacers due to a gap between the column spacers. In this case, the contact area and the contact pressure between the column spacers and the opposing substrate are varied, thereby generating a defect of a cell gap over the whole panel.
Such a problem seriously occurs in large sized LCD panel based on the liquid crystal dispensing method in forming the liquid crystal layer of the LCD panel. This is caused by difficulty in selecting a proper amount of the liquid crystal dispensed on the substrate unlike the liquid crystal dispensing method in which the liquid crystal is injected into the substrate by the pressure difference between the inside of the LCD panel and the outside thereof.